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‘Alien megastructure’ signal may be due to star eating a planet

NASA/JPL-Caltech/T. Pyle

By Leah Crane

Orbiting debris could be making Tabby’s star blink

When you are a messy eater, it can take a long time to clean up after a meal. The slow dimming of Tabby’s star and the sudden dips in its light may be caused by an orbiting cloud of debris left over from when it partially gobbled a planet.

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The star KIC 8462852 rose to prominence in 2015, when a team of astronomers led by Yale’s Tabetha Boyajian (after whom the star is nicknamed) observed a series of abrupt dips in its brightness, in which it dimmed by up to 22 per cent before going back to normal.

There are many ideas about what causes the star’s sporadic blinking, from internal stellar dynamics to swarms of orbiting comets to an enormous alien megastructure.

Now Brian Metzger at Columbia University in New York and his colleagues have a theory that could explain both the brief dips in light and the gradual dimming. The group thinks Tabby’s star is just returning to its natural state – after a large, messy meal.

Planetary crumbs

If Tabby’s star devoured a planet in the past, the planet’s energy would have made the star temporarily brighten, then gradually dim to its original state. The bigger the planet was, the longer the star would take to dim. Depending on the size of the planet, this event could have happened anywhere between 200 and 10,000 years ago.

As the planet fell into its star, it could have been ripped apart or had its moons stripped away, leaving clouds of debris orbiting the star in eccentric orbits. Every time the debris passes between us and the star, it would block some light, making the star seem to blink.

If this is true, these sorts of collisions might be much more common than we expected. “We estimated that if Tabby’s star were representative, something like 10 Jupiters would have to fall into a typical star over its lifetime, or maybe even more,” says Metzger – and that number grows into the thousands if the objects are smaller.

Next time we see the light from Tabby’s star dip, Metzger hopes that astronomers will be able to see signatures of planetary debris passing close to the star. “These transits only last a few days, so when we see one, we have to alert all the telescopes and basically point every telescope we have at Tabby’s star,” he says.

A collision of star and planet explains the behaviour of Tabby’s star well, says Jason Wright at Penn State University in University Park. “This paper puts a merger scenario on the table in a credible way,” he says. “I think this moves it into the top tier of explanations.”